Approximately 1.4 million Americans each year develop skin cancer. The standard regimen for this disease is generally either surgery to remove the lesion and a small amount of surrounding normal tissue, or radiation. The standard dose of radiation is 50 to 60 Gy, delivered in 15 to 30 fractions of about 2.0-3.3 Gy per fraction, to eradicate the lesion through radiation destruction. Both surgery and radiation have similar outcomes. Surgery is often the treatment of choice for skin lesions because the surgeon can, in a single session, remove areas of tissue adjacent to and below the lesion to ensure that the entire lesion has been removed. However, if the skin lesion is located on a cosmetically challenging area, where a scar might form and be visually distracting, or if removal of the lesion would result in removal of tissues that would create a visible deficit, such as a nostril, eye lid or ear lobe, thereby requiring subsequent cosmetic surgery to restore the appearance of the patient, radiation is generally a preferred alternative to surgery.
Additionally, radiation is often the preferred method of treatment for the lower extremity in elderly or diabetic patients. These patients often have vascular insufficiency leading to delayed healing and infection after surgery. Furthermore, patients who opt for irradiation of skin cancer have a non-invasive treatment option.
Generally, patients seeking radiation treatment for skin cancer are irradiated in cancer centers by using 1) the electron beams from an electron linear accelerator that is capable of producing multiple x-rays as well as multiple electron energies, or 2) x-rays (often at energy levels of 50 to 300 keV) that are produced by orthovoltage or superficial x-ray equipment. Both types of equipment require that the patient be irradiated in a shielded room. While both modalities may be used for irradiation of skin cancer, many clinicians feel that electron beams provide for more homogeneous dose distributions and less damage to underlying structures, and are therefore the preferred modality for skin cancer irradiation applications.
Another well-known use of electron beam radiation is in the treatment of invasive cancers. Such cancers include neck nodes in head and neck cancer. Additionally, electron beam radiation may be used as a boost in the treatment of breast cancer.
When skin cancer or invasive cancer is to be irradiated, the radiation is generally delivered in a cancer center. A cancer center is a hospital-based or free-standing radiation therapy facility that uses high energy x-ray or high energy electron beam radiation to treat cancer patients, generally on an out-patient basis. At a cancer center, radiation is produced by linear accelerators that generally weigh several tons (often 7 to 10 tons) and require tons of concrete shielding to contain stray radiation. Thus, cancer centers are almost always located in the basement or ground floor level of a hospital or facility. Cancer centers treat both a) patients with invasive cancers and b) those with superficial cancers, such as skin cancer.
For electron irradiation of skin cancer, typically 15 to 30 fractions of electrons at energies of 6 to 9 MeV are required. For electron treatment of invasive cancer, typically 5 to 20 fractions of electrons at energies of 6 to 15 MeV are required. For invasive cancer, typically 90% of all electron energies used are 12 MeV or lower. Energy control is important in the irradiation of skin lesions because the energy of the incident electron beam is directly related to the depth of penetration of the radiation into the patient. The generally accepted standard for irradiation of humans is that the energy be controlled to at least ±5% with ±3% being more desirable.
Many current radiation therapy devices that produce electrons have an accelerator guide mounted substantially horizontally (parallel to the floor) and use a magnet at the exit of the accelerator guide both to bend the beam into the irradiation plane and to select the appropriate electron energy for irradiation. For one example of such a system, see FIG. 1 of U.S. Pat. No. 4,987,309, issued Jan. 22, 1991. In such systems, the bending magnet is a source of x-ray generation and creates substantial stray radiation. In fact, the bending magnet employed to select the energy that is desired for the irradiation is generally the principal source of stray radiation in conventional accelerators that operate in the electron mode. The energy-defining slits in the magnet produce much radiation, as do adjustable collimators that define the field size. These bending magnet systems are thus unsuitable for use in unshielded or substantially unshielded environments. Thus, a substantially shielded facility is required when a conventional linear accelerator is used to provide multi-energy treatments.
The amount of required protective shielding for a typical bending magnet system operating only in the electron mode has been estimated by some to be the equivalent of a foot of heavy density concrete surrounding the system (e.g., in the walls of the treatment room, the floor and the ceiling). Sometimes such shielding can require as much as 24 to 60 inches of reinforced concrete if the unit is intended to produce both electrons and x-rays for treatment. See e.g. U.S. Pat. No. 6,422,748. In some prior art accelerator designs that do not use bending magnets, the ambient stray radiation will be substantially reduced. However, where a beamstopper is not used, approximately 1 to 1.5 tons of movable shielding must generally be positioned about the patient prior to treatment. This shielding is placed 1) laterally about the patient to protect those outside the room from scattered radiation, and 2) below the treatment table to protect those on the floor below. The shielding is placed so as to comply with current radiation exposure standards for the operation of radiation devices.
Electron linear accelerators have been also developed for delivering intraoperative electron beam radiation therapy (IOERT) in unshielded operating rooms. These systems are designed to operate only in the electron mode and require little additional shielding. One such device, sold by Intraop Medical Corporation of Sunnyvale, Calif. under the product name Mobetron 1000, would be suitable for use with the current inventions. Similar intraoperative electron beam systems and facilities are described in U.S. Pat. No. 5,321,271, issued Jun. 14, 1994, the entire disclosure of which is incorporated herein by reference.
Other IOERT units have high dose per pulse operation, and are therefore not as suitable for applications that require a low dose per treatment. Furthermore, these units use microwave power variation only to change energies. It is difficult to achieve the ±5% energy control desired for applications of the instant invention when using such a method for energy variation. Some of these other IOERT units are designed to use 80 to 100 cm-long applicator cones. The size of these cones may make their use for the instant invention impractical. Finally, these units generally require the use of mobile shielding to operate within allowed radiation exposure limits in an unshielded environment.
Irradiation of skin cancer may be accomplished at a facility with a conventional electron linear accelerator, similar to the ones used in cancer centers. These devices, however, generally require a heavily shielded concrete vault to protect personnel from stray radiation. Installation or construction of a concrete vault weighing several tons, and the space occupied by such a radiation vault is impractical for smaller facilities, such as most dermatology practices.